Progressive power lenses are used to correct the vision of people suffering from presbyopia. Presbyopia is a condition in which the eye loses some or all of its ability to accommodate, i.e. to focus sharply at nearby objects. The curvature of the lens of the eye changes as the eye focuses on objects at different distances from the eye. As people age, their eyes become less elastic and therefore can change the curvature of the lens only to a certain degree, if at all. An individual's prescription for a progressive power lens includes a distance-vision optical power (a spherical correction) and an add power (the change in optical power required for near vision, also a spherical correction). The distance vision correction may be designed for far distances, as in driving, for example, or for closer distances, as in computer use, for example. The prescription may also include cylinder and axis for correcting astigmatism of the individual. An astigmatic presbyope is a person whose prescription includes a distance-vision optical power, a non-zero add power, a non-zero cylinder and an axis.
The standard business practice in the progressive power spectacle lenses market is to design and manufacture progressive power semi-finished spectacle lens blanks. One surface (typically the front surface) of a progressive power semi-finished spectacle lens blank is a finished progressive surface, and the other surface (typically the back surface) is unfinished and may be spherical. The progressive surface is designed to have a particular base curve and add power. The base curve, is the spherical curve of the distance-vision region of the progressive surface. The add power is the difference in optical power between the near-vision region of the progressive surface and the distance-vision region.
A progressive power semi-finished spectacle lens blank having a particular base curve and a particular add power is suitable for a range of distance-vision optical powers, the range being dependent for example, on the material and refractive index of the semi-finished lens blank. For example, the Shamir Genesis™ lens available from Shamir Insight, Inc. in San Diego, Calif., is made from plastic with an index of 1.6. Semi-finished spectacle lens blanks are available with a base curve of 1.5 D, 3.0 D, 5.0 D, 6.5 D and 7.5 D, and with an add power from +0.75 D to +3.50 D, in steps of 0.25 D. The range of distance-vision optical powers for which each semi-finished spectacle lens blank is suitable is summarized in the table below:
Availability Table - Shamir Genesis ™ (plastic 1.6)Base 1.5Base 3.0Base 5.0Base 6.5Base 7.5−11.25−6.00−1.75+1.25+3.50to −6.25to −2.00to +1.00to +3.25to +7.00Different Shamir Genesis™ semi-finished spectacle lens blanks therefore correspond to different combinations of base curve and add power. If the different combinations include five base curves (1.5, 3.0, 5.0, 6.5 and 7.5) and twelve add powers (+0.75, +1.00, +1.25, +1.50, +1.75, +2.00, +2.25, +2.50, +2.75, +3.00, +3.25 and +3.50), then a complete set to cover these combinations would include sixty different progressive power semi-finished spectacle lens blanks.
In general, a manufacturer will typically create a finite set of progressive power semi-finished lens blanks of a design family for a particular material and index. Each progressive power semi-finished lens blank in the set corresponds to a different combination of base curve and add power. The range of distance-vision optical powers for which a semi-finished lens blank of a particular base curve and add power is suitable is determined by the material, index and design.
Optical labs then keep a stock of progressive power semi-finished lens blanks of the design family. When preparing a progressive power spectacle lens for an individual, the lab surfaces and polishes the unfinished surface of the semi-finished spectacle lens blank to fit the individual's prescription. If the individual's prescription does not include astigmatism correction, then the unfinished surface of the semi-finished spectacle lens blank is surfaced and polished so that the prescribed distance-vision optical power is provided by the finished progressive power spectacle lens. The surfacing may involve adjusting the spherical curvature of the unfinished surface. If the individual's prescription includes astigmatism correction, then the unfinished surface is surfaced and polished so that the prescribed distance-vision optical power, the prescribed cylinder and the prescribed axis are provided by the finished progressive power spectacle lens. The surfacing may involve adjusting the spherical curvature of the unfinished surface and adding a toric component to the unfinished surface of the lens.
The design of a progressive surface may require complex balancing of competing interests, and may involve the consideration of light coming from different directions and different distances. One measure of optical performance of a progressive power spectacle lens is the distribution of unwanted astigmatism. The design of the progressive surface may have involved an assumption that the other surface of the lens is spherical. When the progressive surface is paired instead with a toric surface, without taking into account the complex interaction of the toric surface with the progressive surface, this may degrade the optical performance of the resulting lens. For example, using identical progressive power semi-finished spectacle lens blanks to match a prescription of SPHERE: −1.50 D, ADD: +2.00 D and to match a prescription of SPHERE: −1.50 D, ADD: +2.00 D, CYLINDER: −3.00 D, AXIS: 45 degrees may result in progressive power spectacle lenses having significantly different optical performance. In particular, the progressive power spectacle lens matching the prescription of SPHERE: −1.50 D, ADD: +2.00 D, CYLINDER: −3.00 D, AXIS: 45 degrees may have a less desirable distribution of unwanted (residual) astigmatism.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity.